This disclosure generally relates to online systems, and more specifically to synchronizing user activity on external systems to a user of an online system when the user is not logged into the online system.
Users often undertake actions on web pages that are external to an online system, such as a social networking system. Importantly, online systems are often able to identify users that perform actions external to the online system. For example, a user may have previously logged into the online system using a browser, which causes the online system to store a cookie on the browser containing an identifier of the user on the online system (e.g. an online system user ID). Therefore, at a subsequent time point, if a user performs an action on a web page of a partner system (e.g. external to the online system), the online system can access the stored cookie from the browser and identify the user of the online system that is associated with the online system user ID as the user that performed the user activity.
However, users today have many choices in client devices (e.g. mobile phones, tablets, computers, laptops, etc.) and browsers (Chrome™, Internet Explorer™, Firefox™, etc.) that can be used to access the Internet. Therefore, an increasing number of scenarios occur where a user performs a user action external to the online system without having previously logged into the online system. In other words, these users use a browser or a client device to perform actions external to the online system while being logged out of the online system. In this scenario, a cookie may not exist on the browser or the client device, thereby preventing the online system from identifying the user that performed the user action.
The inability of an online system to accurately identify user actions that occur external to the online system leads to several drawbacks. For example, the online system may maintain a social graph that describes associations between its users and entities of interest. Therefore, the online system cannot maintain an accurate and comprehensive understanding of the user's interests in the social graph if the online system is unable to identify user actions that occur external to the online system. Another drawback stemming from this inability is that online systems are unable to accurately track the actions of its users on an external web page that may be caused by the presentation of content items. Thus, online systems may significantly underestimate the performance of a presented content item.